And I'm Renee Montagne. The four Republicans still running for president met last night for their last debate before the South Carolina primary. They sparred over health care, abortion and tax returns. The race in South Carolina is now a dead heat, with Mitt Romney's earlier lead wiped out by a late surge by Newt Gingrich.

And as NPR national political correspondent Mara Liasson reports, the debate in Charleston showed why the race has become so close.

MARA LIASSON, BYLINE: Newt Gingrich's swift rise has been fueled by one thing above all: his forceful performances in the debates. And last night, Gingrich was dominant right from the start. When he got the first question, it was about an explosive television interview with his ex-wife, Marianne.

(SOUNDBITE OF DEBATE)

JOHN KING: In it she says that you came to her in 1999, at a time when you were having an affair. She says you asked her, sir, to enter into an open marriage. Would you like to take some time to respond to that?

NEWT GINGRICH: No, but I will.

(SOUNDBITE OF APPLAUSE AND CHEERING)

LIASSON: Gingrich was ready with the same response he's used before in debates to great effect - an attack on the liberal media.

(SOUNDBITE OF DEBATE)

GINGRICH: I think the destructive, vicious, negative nature of much of the news media makes it harder to govern this country, harder to attract decent people to run for public office. And I am appalled that you would begin a presidential debate on a topic like that.

(SOUNDBITE OF CHEERING)

LIASSON: The audience went wild and gave Gingrich a standing ovation.

Mitt Romney was also ready with an aggressive defense of his work as a private investment banker. He's been criticized by his opponents for practicing vulture capitalism.

MITT ROMNEY: I'm someone who believes in free enterprise. I think Adam Smith was right. And I'm going to stand and defend capitalism across this country, throughout this campaign. I know we're going to hit it hard from President Obama, but we're going to stuff it down his throat and point out it is capitalism and freedom that makes America strong.

(SOUNDBITE OF CHEERING AND APPLAUSE)

LIASSON: Rick Santorum came into the debate with a victory of sorts. Yesterday, Iowa's GOP officials gave Santorum a 34-vote lead over Mitt Romney in the caucuses - a race they said nobody won. But it may have come too late for Santorum, who's been stuck in third place here, fighting with Newt Gingrich for conservative votes. Last night, Santorum attacked Gingrich's character, accusing him of grandiosity.

(SOUNDBITE OF DEBATE)

RICK SANTORUM: Newt's a friend. I love him. But at times you've just got, you know, sort of that, you know, worrisome moment that something's going to pop. And we can't afford that in a nominee. We need someone - I'm not the most flamboyant, and I don't get the biggest applause lines here. But I'm steady. I'm solid. I'm not going to go out and do things that you're going to worry about. I'm going to...

GINGRICH: You're right. I think grandiose thoughts. This is a grandiose country of big people doing big things. And we need leadership prepared to take on big projects.

(SOUNDBITE OF CHEERING AND APPLAUSE)

LIASSON: Romney's campaign is worried about Gingrich's rise. It's airing new attack ads against him and it's held daily press conference calls to bash the former speaker for unreliable leadership. Romney's frustration with Gingrich showed last night.

(SOUNDBITE OF DEBATE)

ROMNEY: Mr. Speaker, it was - you talk about all the things you did with Ronald Reagan and the Reagan revolution and the jobs created during the Reagan years and so forth. I mean, I looked at the Reagan diary. You're mentioned once in Ronald Reagan's diary.

LIASSON: Romney's had a difficult week. There have been news reports about his bank accounts in the Cayman Islands and the fact he pays 15 percent on his taxes. And his ambivalence about releasing his own tax returns. He got some support on that front from Ron Paul, who's also refused to release his returns.

RON PAUL: I don't have an intention of doing it, but for a different reason. I'd probably be embarrassed to put my financial statement up against their income.

(SOUNDBITE OF LAUGHTER)

PAUL: And I don't want to be embarrassed because I don't have a greater income.

(SOUNDBITE OF APPLAUSE)

LIASSON: Romney's known for a long time he'd be asked about his taxes, yet he still hasn't come up with a definitive answer. Asked if he would do what his father did when he ran for office and released many years of returns, Romney answered with a tight smile and got a round of boos.

KING: When you release yours, will you follow your father's example?

ROMNEY: Maybe.

(SOUNDBITE OF LAUGHTER)

ROMNEY: You know, I don't know how many years I'll release. I'll take a look at what the - what our documents are...

(SOUNDBITE OF BOOS)

ROMNEY: ...and I'll release multiple years. I don't know how many years, and - but I'll be happy to do that. Let me tell you, I know there are some who are very anxious to see if they can't make it more difficult for a campaign to be successful. I know the Democrats want to go after the fact that I've been successful. I'm not going to apologize for being successful. And...

(SOUNDBITE OF APPLAUSE)

ROMNEY: Romney says his taxes are, quote, "carefully managed," but his demeanor was defensive, suggesting he considers this issue to be a potential liability. Newt Gingrich thinks so too.

(SOUNDBITE OF DEBATE)

GINGRICH: If there's anything in there that is going to help us lose the election, we should know it before the nomination. And if there's nothing in there...

(SOUNDBITE OF CHEERING AND APPLAUSE)

GINGRICH: If there's nothing in there, why not release it?

LIASSON: Gingrich released his own returns last night. They show he paid a rate of about 30 percent - double the 15 percent Romney pays. An average of South Carolina polls show the race is a dead heat, with Romney holding on to a slim 1.2 percent lead. If Gingrich's surge continues, he could upset Romney in South Carolina - a state that's always chosen the candidate who's gone on to become the nominee. The Republican nominating battle appears to have finally become a two-man race. Mara Liasson, NPR News, Charleston. Transcript provided by NPR, Copyright NPR.